I’ve just learned that different parts of the ocean can have different levels. How does this work? If I drop water into a cup, the entire level of water in the cup rises. How is this not true for the ocean? Are their just parts of the ocean that are uneven? Where do they meet?
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The biggest changes in levels are due to local differences in the earth’s gravity. Some places just happen to have concentrations of denser-than-average rocks near the surface and that makes gravity a little stronger there. Where this happens in the ocean, the sea level is higher because the higher gravity is attracting slightly more water.
The coordinate system used by GPS (WGS84) is based on a perfectly smooth, mathematical ellipsoid. Differences in gravity mean that actual sea level is different from GPS sea level by up to roughly 100 metres, depending where you are on the earth (see [the map](https://www.researchgate.net/figure/Difference-in-meters-between-the-WGS84-spheroid-and-the-approximate-mean-sea-level-as_fig1_224239584)). This is a much larger difference than that caused by tides, air pressure and winds, which all change daily.
Local temperature differences don’t cause a change in local levels because the water is free to flow. Globally, a change in the average ocean temperature would cause a global change in levels but this is a very slow process.
> If I drop water into a cup, the entire level of water in the cup rises.
The same thing happens in the ocean. If you could add enough water in one place in the world then, within hours or days, average water levels would rise globally. It’s just that the sea level would start out and finish uneven.
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